1. Field of the Invention
The present invention relates to Video Telephony (VT) in a Mobile Station (MS) and a mobile communication system. More particularly, the present invention relates to a technique for increasing a call success ratio for VT in an MS and a mobile communication system.
2. Description of the Related Art
When Video Telephony (VT) is performed in mobile communication systems, Mobile Stations (MSs) at both ends of a communication link mutually exchange video stream data and may both use an H.245 protocol to control the stream data.
In the H.245 protocol, control messages are transmitted and received to mediate performance and a channel between the MSs. The control messages are used to perform various functions for MS capability exchange, opening and closing logical channels, mode request, flow control, etc.
In addition, the control messages may be used for master/slave determination and delay time determination.
The H.245 protocol also manages Quality of Service (QoS) control and negotiation for compatibility between the MSs. The QoS control includes frame/bit-rate adjustment, video size adjustment, fast update request, etc.
The VT is achieved when the negotiation process is performed by exchanging signaling messages over the protocol, and thereafter, actual video and audio data are transmitted according to a preset data format.
In addition to the exchange of signaling messages using the protocol, service providers of the mobile communication system require transmission of the video data of a counterpart user within several seconds. In particular, for each service provider, it is recommended that they be capable of achieving an average VT success ratio of over 90%.
When the signaling messages are exchanged, timer and counter information defined in international standards and service providers' standard documents are defined and used. The timer and counter information is used to handle errors and exceptions which may occur while dealing with requests and acknowledges generated in the process of exchanging the signaling messages.
FIG. 1 illustrates a conventional signaling procedure.
Referring to FIG. 1, a signaling message is exchanged in five steps according to a standard, wherein each step uses a fixed timer and a fixed retry counter.
First, an MS A 110 and an MS B 120 transmit VT call setup and VT call connection messages to perform VT in steps A and B. Thereafter, a Master/Slave Determination (MSD) process is performed. For the MSD process, a timer is set to 12.6 sec, and the retry counter is set to 3 times.
If the timer and the counter are fixedly defined irrespective of a Radio Signal Strength (RSS), they may cause a failure to operate within a required time resource in a VT system.
If the retry is performed 3 times in steps C to G as the timer expires at 12.6 sec, a time consumed in the MSD process is 37.8 sec, resulting in a VT connection failure.
In a worse case scenario, theses five steps C to G may be repeated two times to attempt to establish a connection. In this case, a time required for connection is 126 sec (i.e., (12.6 sec*2 times)*5 steps=126)).
Accordingly, if the timer and the counter are fixedly used irrespective of the RSS in the signaling procedure, the retry counter increases when the RSS is low. As a result, significant time delay occurs, which is a primary cause of a call failure.